Ibaizabal is considered by some to be an equally important river, so the whole system is often called Nervión-Ibaizabal and the final tract just "Estuary of Bilbao". The basin area is.
Naming
Despite its current success in Spanish use, the traditional name of the stretch between the estuary and Basauri, where both Ibaizabal and Nervión merge, is Ibaizabal, according to historical evidence tracing back to the eleventh century and up to the twentieth. Charles Frederick Henningsen, who travelled across the Basque Country during the First Carlist War, calls Ibaizabal to the river in Bilbao. The literary writer from Bilbao Miguel de Unamuno as well as mayor of Bilbao Javier Ybarra refer to it as Ibaizabal. Geological and hydrological data also seem to support that assumption.
History and economy
From ancient times, the Orduña pass that links Spain's inner plateau with the Nervión valley has had a great economic impact. During the Middle Ages it served as a natural border between the Biscay lordship and Castile counties, entry taxes were collected there. Through the centuries, the Nervión valley was a vital communication corridor for Biscay, and the main railroad from Bilbao to the rest of Spain was constructed following the river in 1870. At the same time, Bilbao developed a great commercial activity, with its seaport being the most important in the north coast of Spain. The port's activity relied totally in the navigability of the river in its final. So engineer Evaristo Churruca developed an enormous project that would solve the traditional navigation problems of the river. The estuary divided the population of the Greater Bilbao; the Left Bank was occupied by factories and their workers, the Right Bank was a residential area for the factory owners. Now the Nervión river has lost economic importance, the navigable part of the river is being abandoned in favour of the external port, to allow urban development to take place in the banks of the river after the dismantlement of the industry. The quality of the water has improved through refuse water cleaning and the disappearance of the most polluting industries.
Environment
After a century of intense industrial activity the river, especially the final, was ecologically dead with oxygen levels 20 percent below the norm. This made the Nervión one of the most polluted rivers in the world. In 1990 the local authorities launched a plan to overcome this situation, resulting in a constant improvement of the environmental status that after 15 years has transformed the Nervión in to a clean river, full of aquatic life.
